A spokesman for Lashkar-e-Jhangvi, a Sunni group, claimed responsibility for the bomb, which caused casualties in Quetta's main bazaar, a school and a computer center. Police said most of the victims were Shiites.

Burned school bags and books were strewn around.

Early Sunday, a Pakistani police official said that the blast killed 81 people, with many of the critically injured dying overnight, The Associated Press reported.

"The explosion was caused by an improvised explosive device fitted to a motorcycle," said Wazir Khan Nasir, deputy inspector general of police in Quetta.

"This is a continuation of terrorism against Shiites."

"I saw many bodies of women and children," said an eyewitness at a hospital. "At least a dozen people were burned to death by the blast."

Most Western intelligence agencies have regarded the Pakistani Taliban and al Qaeda as the gravest threat to nuclear-armed Pakistan, a strategic U.S. ally.

But Pakistani law enforcement officials say Lashkar-e-Jhangvi has become a formidable force.

Tensions

Banaras Khan / AFP - Getty Images

Pakistani paramedics work over the dead bodies of bomb blast victims at a hospital in Quetta on February 16, 2013. A remote-controlled bomb targeting Shiite Muslims killed 47 people including women and children and wounded more than 200 in Pakistan's insurgency-hit southwest on Saturday, police and officials said. AFP PHOTO/Banaras KHANBANARAS KHAN/AFP/Getty Images

Last month the group said it carried out a bombing in Quetta that killed nearly 100 people, one of Pakistan's worst sectarian attacks. Thousands of Shi'ites protested in several cities after that attack.

Pakistani intelligence officials say extremist groups, led by Lashkar-e-Jhangvi, have escalated their bombings and shootings of Shiites to trigger violence that would pave the way for a Sunni theocracy in U.S.-allied Pakistan.

More than 400 Shiites were killed in Pakistan last year, many by hitmen or bombs, and the perpetrators are almost never caught. Some hardline Shi'ite groups have hit back by killing Sunni clerics.

The growing sectarian violence has hurt the credibility of the government, which has already faced criticism ahead of elections due in May for its inability to tackle corruption and economic stagnation.

The schism between Sunnis and Shiites developed after the Prophet Muhammad died in 632 when his followers could not agree on a successor.

Emotions over the issue are highly potent even today, pushing some countries, including Iraq five years ago, to the brink of civil war.

Pakistan is nowhere near that stage but officials worry that Sunni extremist groups have succeeded in dramatically ratcheting up tensions and provoking revenge attacks in their bid to destabilize the country.

Sixty-four people including school children died on Saturday in a bomb attack carried out by extremists from Pakistan's Sunni Muslim majority. NBC's Lester Holt reports.